This invention relates generally to optical communication systems and, more particularly, to techniques for phase modulation of an optical carrier signal. The established method for phase modulation of optical signals in a phase-shift-keying (PSK) modulation system is to use an electro-optical (EO) modulator with a polarizer. The EO modulator method employs a property of modulation theory: that a double sideband suppressed carrier (DSB-SC) signal, obtained by ampliponent, modulation by .+-.1 with a supressed carrier component is equivalent to shifting the carrier .+-..pi.2 in phase. Thus, phase modulation can be obtained by amplitude-modulating the carrier and ensuring that the carrier frequency component is suppressed.
Another known method of phase modulation of laser diodes avoids the use of an EP modulator and involves injection locking with a second laser diode. This was described in Electronics Letters, 18, 5, pp. 210-22, 1982. However, this method requires two laser diodes and is limited to modulation bandwidths less than half of the injection locking bandwidth. Moreover, it has complexities associated with maintaining injection locking of the two laser diodes.
The use of PSK for modulation of optical signals has approximately a 6 dB (decibels) advantage over FSK (frequency shift keying), but the use of an EO modulator imposes a penalty of approximately 3-4 dB, which practically negates this advantage. Further, the use of an EO modulator poses additional problems of reduced reliability, and increased bulk and weight, all of which are of concern in many applications.
It will be appreciated from the foregoing that there is still a significant need for an alternative approach to direct phase modulation of laser diodes used in optical communication systems. The present invention is directed to this end.